Electrostatic loudspeaker



June 18, 1968 E. P. PEABODY 3,389,226

ELECTROSTATIC LOUDSPEAKER Filed Dec. 29, 1964 7 FIG. a P

s 53 5] l3 TO OTHER 5 25 Umns INVENTOR EARL P. PEABODY,

BY \wamy HIS ATTORNEY..

United States Patent 3,389,226 ELECTRGSTATIE LOUDSPEAKER Earl R Peabody,Los Gatos, (Ialih, assignor to General Electric Company, a corporationof New York Filed Dec. 29, 1964, Ser. No. 421,972 Claims. (Cl. 179-111)AESTRACT OF THE DKSCLQSURE An improved electrostatic loudspeaker havinga composite vibrating diaphragm composed of two sheet-like membranessupported together at their edges. These membranes are electricallyconductive and arranged to bulge mutually apart in response toelectrostatic forces, thereby rendering the membrane taut, to reducerattle or flutter.

This invention relates to an improvement in electrostatic loudspeakers.More particularly, the invention concerns itself with electrostaticloudspeakers of the type which depend for their action upon a force ofelectrostatic attraction between a movable or flexible diaphragmelectrode and a rigid plate electrode.

Certain prior art electrostatic loudspeakers of the nature referred tohave utilized a diaphragm comprising two membranes mounted at theirperiphery. However, such loudspeakers as these have been found to besomewhat limited in application by either relatively limited frequencyresponse, or inefficiency of operation. Accordingly, an important objectof the present invention is to provide a new and improved electrostaticloudspeaker which utilizes a double membrane diaphragm and provides aWide range of frequency response.

Another object of my invention is to provide an improved electrostaticloudspeaker which involves substantially minimal rattle or fluttering.

Still another important object of my invention is to provide a novelelectrostatic loudspeaker which includes a pair of tensioned membranessupported at their periphery between fixed plates and arranged in such amanner that electrostatic repulsion of the membranes produces aresultant tautness which tends to inhibit rattle or flutter.

A further object of my invention is to provide an improved electrostatictransducer which is characterized by an improved frequency response andis economical to manufacture.

In carrying out my invention, in one form thereof, I provide a pair ofcontiguously disposed sheet-like membranes which are supported togetherat their peripheral edges. Each of these membranes is electricallyconductive, and the membranes are supported between, but insulated from,a pair of fixed spaced-apart plates. A means is provided for applyingaudio frequency signals to the plates. Means is also provided to furnishelectrostatic forces to the membranes with respect to the plates. Withsuch an arrangement, the membranes are allowed to bulge mutually apartfrom each other when electrostatic forces are applied thereto, forrendering the membranes taut so that they do not rattle or flutter. Sucha loudspeaker structure also achieves an efficient frequency response inthe frequency range of 4020,000 cycles per second (c.p.s.). Such afrequency response is achieved with relatively low distortion andwithout the necessity of providing any crossover networks or other moreexpensive form of frequency response smoothing means.

Further aspects of my invention will become apparent hereinafter, andthe specification concludes with claims particularly pointing out anddistinctly claiming the subject matter which I regard as my invention.The invention, however, as to organization and method of operation,together with further objects and advantages thereof,

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may be best understood by reference to the following description, whentaken in conjunction with. the accompanying drawing, in which:

FIG. 1 is a schematic diagram of an electrostatic loudspeaker stage,embodying one form of my invention;

FIG. 2 is a front elevation view of a multiple electrostatic loudspeakerunit including a series of electrostatic loudspeakers constructed inaccordance with my invention;

FIG. 3 is a fragmentary perspective view illustrating part of theloudspeaker unit of FIG. 2; and

FIG. 4 is a fragmentary sectional view taken along the lines 4-4 of FIG.2 to illustrate in further detail the construction of the loudspeakerarrangement of FIG. 2.

Referring first to FIG. 1 of the drawing, there is shown in accordancewtih one form of my invention, by way of example, an electrostaticloudspeaker 11 which comprises a pair of uniformly configured flexiblediaphragms or membranes 13 and 15 effectively sandwiched between twoclosely-parallel fixed plates 17 and 19. The diaphragms 13 and 15 may beconstructed from a plastic sheet material such as Mylar having athickness of 0.250 mil, to thus provide a pair of thin flexiblemembranes. Each of the diaphragms 13 and 15 has a conductive coatingsurrounding it, such as, for example, aluminum. The plates 17 and 19 areof rigid metallic construction and are suitably perforated so that soundcan pass through them. By way of example, the fixed plates 17 and 19 maybe made of perforated screen sheets disposed in parallel relationshipand mounted on a wooden supporting frame.

The diaphragms 13 and 15 are secured in tension between the plates 17and 19 by being clamped together in mutually contiguous relationship anddisposed in spaced apart insulated relationship from the plates 17 and19. As shown schematically in FIG. 1, the diaphragms 13 and 15 are thusdisposed in generally parallel relationship with each other andperipherally supported in tension by means of insulative members 21 and23 which space the diaphragms 13 and 15 from their adjacent respectiveplates 17 and 19.

Turning now to a description of the operation of my new and improvedloudspeaker 11, attention is further directed to FIG. 1. As showntherein, an audio frequency signal voltage is applied to the fixedplates 1'!" and 19 by means of a signal output transformer 27. Moreparticularly, the audio signal from source S is supplied to the primarywinding 29 of the signal output transformer 27. The transformed signalvoltage from transformer secondary 41 is transmitted by conductors 37and 39 from the secondary 41 to the fixed plates 17 and 19,respectively. Conductors 37 and 39 are each connected at one of theirends to the ends of the secondary transformer winding 41 and areconnected at their other ends to the fixed plates 17 and 19, via theshort conductors 47 and 49, respectively. It will be understood by thoseskilled in the art that the signal output transformer 27 may alsoprovide an audio signal to various other loudspeaker units similar tothe schematically illustrated unit 11, as is indicated by the arrows onthe right side of FIG. 1.

The electrostatic loudspeaker unit 11 and any other accompanying units,are polarized by connecting a suitable DC source of relatively hightension between diaphragm terminal 51, which is common to both of thediaphragms 13, 15, and a center tap 53 of the transformer secondary 41.By way of example, a polarizing voltage of between 800 and 1500 volts DChas been found advantageous. It will thus be seen that by means of thepolarizing voltage applied between terminals 51 and 53 the conductivecoatings of the plastic membranes 13 and 15 are polarized with respectto the plates. The tensioned double diaphragms 13 and 15 will thereuponvibrate between plates 17 and 19 due to the varying forces actingbetween the charge on diaphragnis 13 and 15, and the instantaneousvoltages on plates 17 and 19 due to the applied signal.

In view of the DC voltage applied to terminals 51 and 53, plate 17 hasan opposite polarity from its adjacent diaphragm 13, and as a result ofthis opposite polarity the diaphragm 13 is attracted to plate 17. inaddition, the DC voltage applied to terminals 51 and 53 also causes theplate 19, which is Opposite in polarity from its adjacent diaphragm 15,to attract diaphragm 15. Since diaphragms 13 and are of the samepolarity, they thereupon tend to repel each other. The diaphragms 13 and15 thus tend to bow away from each other at their centers, as indicatedin FIG. 1.

It will thus be seen, that my improved diaphragm arrangement helps tokeep both of the membranes 13 and 15 under tension at all times due tothe mutual repulsion of the diaphragms 13 and 15 and the attraction ofeach diaphragm 13 and 15' to its adjacent respective fixed plate 17 and19. Such a structure permits relatively low prestressing of themembranes and a resultant lower resonant frequency. It also tends tokeep the membranes relatively tight against their supporting means,thereby effectively avoiding any tendency of rattling or flutter. Sincethe electrostatic forces are acting in a push-pull fashion, harmonicdistortion is effectively obviated. In addition, controlled air leakageof the volume between the diaphragms may allow for changes in ambientpressure and temperature.

Turning now to FIGS. 2, 3 and 4, as shown therein, I have illustrated anoperative embodiment of my invention which incorporates a series oftwelve electrostatic loudspeaker units 11 arranged in a room divider 61.The illustrated room divider 61 includes a pair of vertically elongatedinsulative frames 63 and 65 (eg. constructed from Wood) which aresecured together in pivotal relationship by a series of hinges 67. Eachof the frames 63 and 65 has the same configuration and includes a seriesof spaced-apart horizontal ribs 69 which extend between and areconnected to elongated inner and outer vertical legs '71 and 73. Asfurther shown in FIGS. 3 and 4, each of the frames 63 and 65 is of twoply construction so that the horizontal ribs 69 and the inner and outerlegs 71 and 73 of each frame consist of two identical parts suitablysecured together, such as by way of example, the two parts of the upperrib 69 illustrated in FIG. 3.

The horizontal ribs 69 are in parallel relationship and are spaced apartin equidistant fashion from each other, as illustrated in PEG. 2, sothat each frame 63 and 65 provides a series of six rectangularlyconfigured apertures or windows 75. In each of the apertures 75, thereis arranged in tension 21 contiguous pair of the aforementionedmembranes 13 and 15. More particularly, the Mylar diaphragms 13 and 15are clamped together between the peripheral sides of each windowframework 75 in the manner suggested by FIG. 4. Thus, as indicated inFIG. 4, the peripheral edges of the two membranes 13 and are sandwichedtogether in contact with each other, between the mating portions of thewindow periphery, such as horizontal rib portions 6% and 6911. A seriesof bolts 77 with nuts 79 (one of which is illustrated by way of examplein FIG. 4) may be utilized to securely clamp the peripheral edges of thetwo membranes 13, 15 to the surrounding portion of the frame window 75.Connections of the leads 47 and 49 (FIG. 1) to the aluminized surfacesof the membranes 13 and 15 may be achieved by means of pressure contact(not shown) at the frame edges.

In the illustrated embodiment of my invention, to provide additionalsupport for the thin fixed plates 17 and 19 between their upper andlower ends, I have also provided a series of horizontal wooden ribs 81which are arranged in parallel spaced-apart fashion and connected to theinner and outer vertical legs 73 and 71.

The screen-like plates 17 and 19 are secured to the Cir frame by meansof contact cement (not shown). Plate 17 is in generally paralleladjacency to membrane 13 and located outwardly therefrom. Plate 19 is ingenerally parallel a iacency to membrane 15 and located outwardlytherefrom. A spacing of 60 mils between each of the plates 17, 19 andthe closest surface of their adjacent membranes 13, 15, has been foundto be particularly desirable. Suitable conductors may be soldered toeach of the plates and brought out through framework for externalconnection to the circuit.

it will be understood by those skilled in the art that suitable grillecloth of fabric or insulative plastic may be readily draped over theroom divider frames 63, after the assembly of the electrostaticloudspeaker units 11 thereto, to help appropriately insulate tie speakerassembly from the operator and also to enhance the attractiveness of theoverall device. it will also be understood that the electrostaticspeaker units 11, or a plurality of room dividers 61 may be connected insuch a manner as to effectively transmit stereophonic sound.

It will now therefore be understood that in accordance :ith the presentinvention I have provided an improved electrostatic loudspeaker whicheffectively utilizes a pair of co-operating membranes arranged in tautfashion so that they do not rattle or flutter. it will be furtherrealized that my improved electrostatic loudspeaker units providerelatively low distortion and full range frequency response without thenecessity of utilizing any crossover networks or the like. It shall beunderstood that my invention provides a simplified electrostaticloudspeaker which is characterized by very efficient and stabilizedoperation. It will also be seen that I have provided an improvedelectrostatic loudspeaker enclosure wherein my loudspeaker units may beeffectively utilized.

While the illustrated diaphragms 13 and 15 are constructed of theplastic sheet material Mylar having a conductive surrounding coating ofaluminum, it will be understood that my invention is not necessarilylimited to such a diaphragm structure as this. For example, it may bedesirable to construct the diaphragms 13 and 15 in such a manner thatonly the inwardly facing and opposite sides of each diaphragm have aconductive coating formed thereon. With such an alternative arrangement,the outwardly facing peripheral edges of each diaphragm would beinsulative and could be placed in contiguous relationship with theperipheral edges of the fixed plates 17 and 19. Such a structure wouldreadily lend itself to economy of manufacture.

it will be further understood that the illustrated diaphragms 13 and 15may be constructed in such a manner that only their outwardly facingsides have a conductive coating formed thereon, and the inwardly facingsurfaces are of the insulative Mylar. With such an arrangement, abracketing type terminal 51 (not shown) could be utilized for connectingthe periphery of the diaphragms 13, 15 to the positive side of thepolarizing source.

While in accordance with the patent statutes, I have described what atpresent are considered to be the preferred embodiments of my invention,it will be obvious to those skilled in the art that various changes andmodifications may be made therein without departing from my invention,and I, therefore, aim in the following claims to cover all suchequivalent variations as fall within the true spirit and scope of thisinvention.

What I claim and desire to secure by Letters Patent of the United Statesis:

1. An electrostatic loudspeaker system comprising first and second outerplate electrodes disposed in parallel relationship, first and secondvibratable diaphragm electrodes of flexible sheet material disposedbetween, spaced from and insulated from said outer plate electrodes,said first and second diaphragm electrodes being disposed in contiguousrelationship, driving means for applying audiofrequency signals to theplate electrodes, and means for continuously applying a polarizingvoltage to said dia- 3,3 5 phragms with respect to said plateelectrodes, said polarizing means causing said diaphragms to bulgemutually apart thereby to render said diaphragms taut for enhancing thestability of operation of said electrostatic loudspeaker system.

2. The loudspeaker system of claim 1 wherein the diaphragm electrodesare constructed of thin plastic material and surrounded by a coating ofa conductive material.

3. The loudspeaker system of claim 1 wherein the inwardly facingsurfaces of said diaphragm electrodes are coated with conductivematerial and the outwardly facing surfaces are insulative.

4. The loudspeaker system of claim 1 wherein the outwardly facingsurfaces of said diaphragm electrodes are coated with conductivematerial and the inwardly facing surfaces are insulative.

5. An electrostatic loudspeaker system comprising a pair of two plysupporting panels arranged in pivoted relationship, each of saidpanels'including a plurality of 20 two-ply window frames, and anelectrostatic loudspeaker unit disposed in each of said window frames,said loudspeaker unit comprising first and second outer plate electrodesdisposed in parallel relationship within said window frame, first andsecond vibratable diaphragm electrodes of flexible sheet materialdisposed between, spaced from and insulated from said outer plateelectrodes, said first and second diaphragm electrodes being disposed incontiguous generally parallel relationship, driving means for applyingaudio-frequency signals to the plate electrodes of each loudspeakerunit, and means for continuously applying a polarizing voltage to saiddiaphragms of each unit with respect to said plate electrodes thereof,said polarizing means causing said diaphragms to bulge mutually apartthereby to render said diaphragms taut for enhancing the stability ofoperation of each electrostatic loudspeaker unit.

References Cited UNITED STATES PATENTS 1,850,855 3/1932 Thomas 179-1113,008,014 11/1961 Williamson et a1. 17911l 3,136,867 6/1964 Brettell179-111 KATHLEEN H. CLAFFY, Primary Examiner. R. P. TAYLOR, AssistantExaminer.

